Telephone with automatic switching between cellular and voip networks

ABSTRACT

A mobile communication device includes, in part, a cellular communication module, a first antenna adapted to receive and transmit data between the mobile communication module and a cellular network, a Wireless Fidelity (Wi-Fi) communication module, a second antenna adapted to receive and transmit data between the Wi-Fi communication module and a VoIP network, a signal monitoring circuit, and a switching circuit adapted to switch an existing communication from the cellular communication module to the Wi-Fi communication module or vice versa.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 USC 119(e) of the filingdate of U.S. provisional application No. 60/534,466, filed on Jan. 6,2004, entitled “Radiotelephone With Automatic Switching Between CellularAnd Wi Fi Networks Using Wi-Fi Signal Strength Values”, the content ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

A small portion of the radio frequency (RF) spectrum is assigned to eachcommunications carrier. The assigned spectrum, therefore, must be usedefficiently in order to allow the maximum number of frequency users tohave access to this spectrum. Multiple access modulation techniques havebeen developed to provide optimum utilizing of the RF spectrum. Examplesof such modulation techniques include time division multiple access(TDMA), frequency division multiple access (FDMA), and code divisionmultiple access (CDMA).

There is a wide variance in the performance of wireless networks. Aconventional wireless cellular network, covers a relatively largegeographical area but provides a relatively low bandwidth. Such wirelessnetworks use regulated portions of the radio spectrum and are shared bymany users. The infrastructure costs of wireless networks are relativelyhigh due to the size and complexity of the cellular network equipment.

Other wireless networks, such as CDMA2000-EV-DO/DV networks, offerhigher bandwidth and enhanced data services, such as web browsing.However, these networks also pack many users into a relatively smallportion of the regulated spectrum. Other types of radio networks areadapted to improve spectral efficiency with increased and smallercoverage areas. For example, an IEEE 802.11x (or Wi-Fi) network maytransmit at speeds up to 11 Mbps using a Direct Sequence Spread Spectrum(DSSS) mode or at speeds up to 54 Mbps using an Orthogonal FrequencyDivision Multiplexing (OFDM) mode.

An access point conforming to an IEEE 802.11x (e.g., IEEE 802.11b)network may cover an area of a few hundred feet in diameter. Each suchaccess point is connected to a larger network (e.g., Internet). In orderto cover larger geographical areas, a relatively large number of IEEE802.11x network access points and a relatively large wire-line back haulnetworks are required. In part, due to the back haul costs, theresulting IEEE 802.11x based network may thus be more expensive to setup and operate than a similarly based wireless network. In other words,many tradeoffs often exist between and among the coverage areas, themaximum bit-rates, and the costs associated with different types ofwireless networks.

Demand for high bandwidth and quality of service (QoS) associated withmobile communication devices with full roaming capability is on therise. One known communication device includes a cellular communicationmodule adapted to enable communication using wireless cellular networksas well as a Wi-Fi communication module adapted to enable communicationusing a Voice over IP (VoIP) protocol. In such devices, to change thecommunication mode from cellular to VoIP or vice versa, the user has tomanually change the device's setting by, e.g., pressing one ore morekeys.

In yet other communication devices known to be under development, toswitch the communication mode from, for example, cellular to VoIP, thecellular network first detects the position of the mobile communicationdevice to determine whether the mobile communication device is in aWi-Fi area. If it so detects, the cellular network sends a switchingsignal to the mobile communication device to enable the communication tocontinue the communication using the VoIP protocol. However, obtainingand maintaining accurate position of many mobile communication devicesconcurrently poses a challenging task. Consequently, in such systems,the switching of the call from cellular to VoIP or vice versa may resultin the loss of the call.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a mobile communication deviceis configured so as to automatically switch a communication that isalready in progress using a wireless cellular network (hereinafteralternatively referred to as cellular network) to a wireless Voice overIP (VoIP) network or vice versa. The mobile communication devices isadapted to include, in part, a cellular communication module, a firstantenna adapted to receive and transmit data between the mobilecommunication module and a cellular network, a Wireless Fidelity (Wi-Fi)communication module, a second antenna adapted to receive and transmitdata between the Wi-Fi communication module and a VoIP network, a signalmonitoring circuit, and a switching circuit adapted to switch anexisting in-progress communication between the cellular communicationmodule and the Wi-Fi communication module. The second antenna andassociated circuitry are maintained in on-states continuously to monitorand detect Wi-Fi signals.

If the mobile communication device is in an in-progress (i.e.,pre-established) communication via its cellular communication module andthrough a cellular network, and the Wi-Fi antenna system detects a Wi-Fisignal having a first predefined level (strength), a timer disposed inthe mobile communication device is activated to establish a first timewindow of a first predefined size. If the Wi-Fi signal level detectedduring the first time window remains equal to or greater than the firstpredefined level, at the expiration of the first time window, theswitching circuit causes the in-progress communication to be switchedfrom its cellular communication module to its Wi-Fi communication moduleand through a VoIP network without losing the in-progress communication.

In some embodiments, upon activating the timer, the Wi-Fi communicationmodule is caused to change state from a sleep mode, during which theWi-Fi communication module consumes relatively small amount of power, toa stand-by mode during which the Wi-Fi communication module consumes anintermediate amount of power. Subsequently, before the communication isswitched to the Wi-Fi communication module, the Wi-Fi communicationmodule is caused to be placed in a full active mode, during which theWi-Fi communication module consumes an amount of power larger than theintermediate amount of power.

If the mobile communication device is in a pre-established communicationvia its Wi-Fi communication module and through a VoIP network, and theWi-Fi antenna system detects that the level of the received Wi-Fi signalis below a second predefined value, the timer is activated to establisha second time window of a second predefine size. If the Wi-Fi signallevel detected during the second time window is equal to or greater thana third predefined value, the pre-established communication via theWi-Fi communication module is maintained without any change. If theWi-Fi signal level detected during the second time window is less thanthe third predefined value, the timer is reset and reactivated toestablish a third time window of a third size. If the Wi-Fi signal leveldetected during the third time window is less than the third predefinedvalue, at the expiration of the third time window, the switching circuitcauses the in-progress communication to be switched from its Wi-Ficommunication module to its cellular communication module and through acellular network without losing the in-progress communication.

In some embodiments, upon activating the timer to establish the secondtime window, the cellular communication module is caused to change statefrom a sleep mode, during which the cellular communication moduleconsumes relatively small amount of power, to a stand-by mode duringwhich the cellular communication module consumes an intermediate amountof power. Subsequently, before the communication is switched to theWi-Fi communication module, the cellular communication module is causedto be placed in a full active mode, during which the cellularcommunication module consumes an amount of power larger than theintermediate amount of power.

In some embodiments, the Wi-Fi communication module is adapted tocommunicate with an access point of a Wi-Fi local are network using an802.11x wireless protocol, and the cellular communication module isadapted to communicate with a base station of a wireless cellularnetwork using any one of GSM, CDMA, or CDMA2000 protocols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified high-level block diagram of a mobilecommunication device, in accordance with one embodiment of the presentinvention.

FIG. 2 shows Wi-Fi signal threshold level and timing window used todetermine whether to switch a communication from the cellularcommunication module to the Wi-Fi communication module of thecommunication device of FIG. 1, in accordance with one embodiment.

FIG. 3 shows Wi-Fi signal threshold levels and timing windows used todetermine whether to switch a communication from the Wi-Fi communicationmodule to the cellular communication module of the communication deviceof FIG. 1, in accordance with one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a mobile communication deviceis configured so as to automatically switch an existing communicationfrom a wireless cellular network (hereinafter alternatively referred toas cellular network) to a wireless Voice over IP (VoIP) network, or toswitch an existing communication from a VoIP network to a cellularnetwork. It is understood that the Wireless Fidelity (Wi-Fi) signals, asdefined, for example, in IEEE 802.11x standards or other equivalentstandards may be used to communicate with a Voice over IP (VoIP)network. The mobile communication device is adapted to include, in part,a cellular communication module, a first antenna adapted to receive andtransmit data between the mobile communication module and a cellularnetwork, a Wi-Fi communication module, a second antenna adapted toreceive and transmit data between the Wi-Fi communication module and aVoIP network, a signal monitoring circuit, and a switching circuitadapted to switch an existing in-progress communication between thecellular communication module and the Wi-Fi communication module. Thesecond antenna and associated circuitry are maintained in on-statescontinuously to monitor and detect Wi-Fi signals.

It is understood the wireless cellular network includes, in part, amultitude of base stations. Each such base station is adapted tocommunicate with the mobile communication device when the mobilecommunication device is located within the coverage area of the basestation via RF signals carried over cellular network. It is alsounderstood that each such coverage area is defined by an area centeredat the base station and having a radius of, e.g., several miles. It isfurther understood that a Wi-Fi network may include, in part, amultitude of access points. Each such access point is adapted tocommunicate with the mobile communication device, when the mobilecommunication device is located within the coverage area of the accesspoint, via VoIP packets. It is also understood that the mobilecommunication device may also include blocks adapted for computation andthus be a communication/computation device.

If the mobile communication device is in an in-progress (i.e.,pre-established) communication via its cellular communication module andthrough a cellular network, and the Wi-Fi antenna system detects a Wi-Fisignal having a first predefined level (strength), a timer disposed inthe mobile communication device is activated to establish a first timewindow of a first predefined size. If the Wi-Fi signal level detectedduring the first time window remains equal to or greater than the firstpredefined level, at the expiration of the first time window, theswitching circuit causes the in-progress communication to be switchedfrom its cellular communication module to its Wi-Fi communication moduleand through a VoIP network without losing the in-progress communication.

in some embodiments, upon activating the timer, the Wi-Fi communicationmodule is caused to change state from a sleep mode, during which theWi-Fi communication module consumes relatively small amount of power, toa stand-by mode during which the Wi-Fi communication module consumes anintermediate amount of power. Subsequently, before the communication isswitched to the Wi-Fi communication module, the Wi-Fi communicationmodule is caused to be placed in a full active mode, during which theWi-Fi communication module consumes an amount of power larger than theintermediate amount of power.

If the mobile communication device is in a pre-established communicationvia its Wi-Fi communication module and through a VoIP network, and theWi-Fi antenna system detects that the level of the received Wi-Fi signalis below a second predefined value, the timer is activated to establisha second time window of a second predefine size. If the Wi-Fi signallevel detected during the second time window is equal to or greater thana third predefined value, the pre-established communication via theWi-Fi communication module is maintained without any change. If theWi-Fi signal level detected during the second time window is less thanthe third predefined value, the timer is reset and reactivated toestablish a third time window of a third size. If the Wi-Fi signal leveldetected during the third time window is less than the third predefinedvalue, at the expiration of the third time window, the switching circuitcauses the in-progress communication to be switched from its Wi-Ficommunication module to its cellular communication module and through acellular network without losing the in-progress communication.

In some embodiments, upon activating the timer to establish the secondtime window, the cellular communication module is caused to change statefrom a sleep mode, during which the cellular communication moduleconsumes relatively small amount of power, to a stand-by mode duringwhich the cellular communication module consumes an intermediate amountof power. Subsequently, before the communication is switched to theWi-Fi communication module, the cellular communication module is causedto be placed in a full active mode, during which the cellularcommunication module consumes an amount of power larger than theintermediate amount of power.

FIG. 1 is a simplified high-level block diagram of a mobilecommunication device 100, in accordance with one embodiment of thepresent invention. Mobile communication device 100, which is adapted toautomatically switch communication between cellular and VoIP networks,is shown as including, in part, a cellular communication module(hereinafter alternatively referred to as cellular module) 3 coupled toa cellular antenna 1, a Wi-Fi communication module (hereinafteralternatively referred to as Wi-Fi module) 4 coupled to a Wi-Fi antenna2, an audio/video amplifier 5, a network switch unit 6, a timer unit 7,a Wi-Fi signal level monitor 8, a microphone 20, a speaker 21, and adisplay monitor 22. Mobile communication device 100 is adapted toestablish and maintain communication via either a cellular module 3,through a wireless network (not shown), and/or via Wi-Fi module 4,through a VoIP network (not shown). Cellular module 3 further includes atransceiver 30 adapted to transmit signal to and receive signal from acellular network. Wi-Fi module 4 further includes a transceiver 32adapted to transmit signal to and receive signal from a VoIP network.Depending on the level of detected Wi-Fi signal emitted from a Wi-Fiaccess point, a call initially established via cellular module 3 may beswitched to be handle by Wi-Fi module 4, or a call initially establishedvia Wi-Fi module 4 may be switched to be handled by cellular module 3.

Assume that mobile communication device 100 is in communications with acellular network and is entering the coverage area of a Wi-Fi accesspoint adapted to transmit and receive Wi-Fi signals. As is known, aWi-Fi access point may be used to gain access to a VoIP network. Wi-Fiantenna 2 together with Wi-Fi module 4 and Wi-Fi signal monitor 8continuously monitor for to detect Wi-Fi signals. If a detected Wi-Fisignal level is greater than a predefined threshold value V_(th1), Wi-Fisignal level monitor 8 activates timer 7 via signal line 10 and sends awake-up signal to network switch unit 6 via signal line 12, thereby tochange the state of network switch unit 6 from a sleep mode, duringwhich network switch unit consumes relatively small power, to a stand-bymode, during which network switch unit consumes an intermediate amountof power. Network switch unit 6, in turn, supplies wake-up switchingsignals to cellular module 3 via signal line 15 and to Wi-Fi module 4via signal line 13. This causes phone cellular module 3 and Wi-Fi module4 to activate their respective switching modules. The detected Wi-Fisignal level may be determined, for example, by taking multiple samplesof the incoming Wi-Fi signal(s) and computing a signal level from thesesamples. In one embodiment, an average of the amplitudes/phases of thesampled signals may be used to detect the Wi-Fi signal level.

If the detected Wi-Fi signal level is greater than V_(th1), timer unit 7is activated to establish a first time window of a first predefined sizeT₁, as shown in FIG. 2. During time window T₁, Wi-Fi antenna 2 togetherwith Wi-Fi module 4 and Wi-Fi signal monitor 8 continue to monitor anddetect the level of received Wi-Fi signal. If the Wi-Fi signal leveldetected during time window T₁ remains equal to or greater than V_(th1),at the expiration of time window T₁, timer unit 7 sends a cellulartear-down signal and a Wi-Fi link-up signal to the network switch unit 6via signal line 11. In response, network switch unit 6 sends thetear-down signal to cellular module 3 via signal line 15, and sends thelink-up signal to Wi-Fi module 4 via signal line 13. Network switch unit6 also instructs audio/video amplifier 5, via signal line 14, togenerate an audio/video alert signal. The generated audio alter signalis subsequently reproduced by speaker 21 via signal line 18, and thegenerated video alert signal is subsequently reproduced by displaymonitor 22 via signal line 23. The audio/video alert tones are adaptedto notify the mobile communication device user of a network switch fromcellular to Wi-Fi.

Cellular module 3 is adapted to terminate connection to the cellularnetwork and to switch off connection to audio/video amplifier 5 afterreceiving the tear-down signal. Wi-Fi module 4 is adapted to activateconnection to VoIP network and to switch on connection to audio/videoamplifier 5—for passing voice signal to audio amplifier 5 and videosignal to display monitor 22—after receiving a Wi-Fi link-up signal. Thepreviously established communication link is thus continueduninterrupted via the VoIP network and through loudspeaker 21,microphone 20 and display monitor 23.

Assume that mobile communication device 100 is in communications with aVoIP network and may be leaving the coverage area of the Wi-Fi accesspoint. Wi-Fi antenna 2 together with Wi-Fi module 4 and Wi-Fi signalmonitor 8 continuously monitor for to detect Wi-Fi signals. If thedetected Wi-Fi signal level falls below a second predefined thresholdvalue V_(th2), Wi-Fi signal level monitor 8 activates timer 7 and sendsa wake-up signal to network switch unit 6 to change the state of networkswitch unit 6 from the sleep mode to stand-by mode. Network switch unit6, in turn, supplies wake-up switching signals to cellular module 3 andto Wi-Fi module 4 to enable these modules to activate their respectiveswitching procedures.

Once activated, timer unit 7 establishes a second time window of asecond predefined size T₂, as shown in FIG. 3. During time window T₂,Wi-Fi antenna 2 together with Wi-Fi module 4 and Wi-Fi signal monitor 8continue to monitor and detect the level of received Wi-Fi signal. Ifthe Wi-Fi signal level detected during time window T₂ is equal to orgreater than a third predefined threshold value V_(th3), where V_(th3)is smaller than V_(th2), the previously established VoIP communicationcontinues without switching.

If the Wi-Fi signal level detected during time window T₂ is smaller thanV_(th3), at the expiration of time window T₂, timer unit 7 is reset andactivated to establishes a third time window of a third predefined sizeT₃, where T₃ is smaller than T₂. If the Wi-Fi signal level detectedduring time window T₃ is equal to or greater than V_(th3), thepreviously established VoIP communication continues without switching.

If the Wi-Fi signal level detected during time window T₃ is less thanV_(th3), at the expiration of time window T₃, timer unit 7 sends a Wi-Fitear-down signal and a cellular link-up signal to network switch unit 6.In response, network switch unit 6 sends the tear-down signal to Wi-Fimodule 3, and sends the link-up signal to cellular module 4. Networkswitch unit 6 also instructs audio/video amplifier 5 to generate anaudio/video alert signals. The generated audio alter signals issubsequently reproduced by speaker 21, and the generated video alertsignal is subsequently reproduced by display monitor 22. The audio/videoalert tones are adapted to notify the mobile communication device userof a network switch from Wi-Fi to cellular

Wi-Fi module 4 is adapted to terminate connection to the VoIP networkand to switch off connection to audio/video amplifier 5 after receivingthe tear-down signal. Cellular module 4 is adapted to activateconnection to the cellular network and to switch on connection toaudio/video amplifier 5 after receiving a Wi-Fi link-up signal. Thepreviously established communication link is thus continueduninterrupted and through loudspeaker 21, microphone 20 and displaymonitor 23.

In accordance with some embodiments, if the mobile communication devicedetects both an Wi-Fi signal from an access point and a cellular signalfrom a mobile cellular base station before establishing a communicationlink, the mobile communication device first attempts to establishcommunication with the Wi-Fi access point using Wi-Fi module 4.

The above embodiments of the present invention are illustrative and notlimiting. Various alternatives and equivalents are possible. It isunderstood that the functionality associated with any blocks describedabove may be centralized or distributed, whether locally or remotely. Itis also understood that one or more blocks of each mobile communicationdevice may be performed by hardware, firmware or software, or somecombinations thereof. The invention is not limited by the type ofcellular network, e.g., CDMA, GSM, otherwise used to carrycommunication. Nor is the invention limited by the VoIP network. Theinvention is not limited by the Wi-Fi signals, such as those defined byIEEE 802.11x, where x may be a, b, g, or WiMAX used to carry VoIPcommunication. The invention is not limited by the type of integratedcircuit(s) in which the present invention may be disposed. Nor is theinvention limited to any specific type of process technology, e.g.,CMOS, Bipolar, or BICMOS that may be used to manufacture the presentinvention. Other additions, subtractions or modifications are obvious inview of the present invention and are intended to fall within the scopeof the appended claims.

1. A mobile communication device comprising: a cellular communicationmodule adapted to communicate with a wireless cellular network; a Wi-Ficommunication module adapted to communicate with an access pointassociated with a Wi-Fi network using a VoIP protocol; a switchingcircuit adapted to automatically switch operation between the cellularcommunication module and the Wi-Fi communication module; a timer; and aWi-Fi signal monitor; wherein if the Wi-Fi signal monitor detects that alevel of a Wi-Fi signal is greater than a first predefined thresholdvalue V_(th1), the timer is activated to begin a first time window of afirst predefined size T₁, wherein if during T₁ the Wi-Fi signal monitordetects that the level of the Wi-Fi signal remains equal to or greaterthan V_(th1), at the expiration of the first time window the switchingcircuit sends a tear-down signal to the cellular communication moduleand a link-up signal to the Wi-Fi communication module, wherein saidtear-down signal causes the cellular communication module to discontinuehandling a previously established cellular communication and whereinsaid link-up signal causes the Wi-Fi communication module to handle thepreviously established cellular communication via a VoIP network,wherein if the mobile communication device detects both a Wi-Fi signalfrom the access point and a cellular signal from the wireless cellularnetwork before establishing a communication link, the mobilecommunication device attempts to establish communication with the accesspoint before attempting to establish communication with the wirelesscellular network.
 2. The mobile communication device of claim 1 whereinsaid wireless cellular network is selected from a group consisting ofGSM, CDMA, and CDMA2000 wireless cellular networks.
 3. The mobilecommunication device of claim 2 wherein said Wi-Fi network is selectedfrom a group consisting of an IEEE 802.11x, Wi-Fi, and WiMAX networks.4. (canceled)
 5. (canceled)
 6. (canceled)
 7. The mobile communicationdevice of claim 1 wherein the level of the Wi-Fi signal is defined by aplurality of samples the of received Wi-Fi signal.
 8. (canceled) 9.(canceled)
 10. (canceled)
 11. A method comprising: detecting a firstWi-Fi signal level; activating a timer to establish a first time windowof a first predefined size T₁ if the detected first Wi-Fi signal levelis greater than a first predefined threshold value V_(th1); detecting asecond Wi-Fi signal level during the first time window; andautomatically switching a communication in progress via a cellularwireless network to a communication via a VoIP network if the seconddetected Wi-Fi signal level remains greater than V_(th1).
 12. The methodof claim 11 wherein said wireless cellular network is selected from agroup consisting of GSM, CDMA, and CDMA2000 wireless cellular networks.13. The method of claim 11 wherein said VoIP network is in communicationwith a network selected from a group consisting of an IEEE 802.11x,Wi-Fi and WiMAX networks.
 14. The method of claim 11 wherein each ofsaid first and second Wi-Fi signal levels is defined by a respectiveplurality of samples of received Wi-Fi signal.
 15. (canceled) 16.(canceled)
 17. The mobile communication device of claim 1 wherein eachof said first and second Wi-Fi signal levels is defined by a respectiveaverage of a plurality of samples of received Wi-Fi signal.
 18. Themobile communication device of claim 1 wherein only one of the cellularand Wi-Fi communication modules is connected to the wireless cellularnetwork or the WiFi network unless the switching circuit is in theprocess of switching operation between the cellular communication moduleand the Wi-Fi communication module.
 19. The method of claim 11 furthercomprising: sending a tear down signal from a switching circuit to acellular communication module and a link-up signal from the switchingcircuit to a Wi-Fi communication module, wherein the switching circuit,the cellular communication module, and the Wi-Fi communication moduleare comprised within a single mobile communication device.
 20. A mobilecommunication device comprising: a cellular communication module tocommunicate with a wireless cellular network; a Wi-Fi communicationmodule to communicate with an access point associated with a Wi-Finetwork using a VoIP protocol; a Wi-Fi signal monitor; a timer that isactivated if the Wi-Fi signal monitor detects a Wi-Fi signal greaterthan a first predefined threshold value V_(th1); and a switching circuitthat executes a handoff between the cellular communication module andthe Wi-Fi communication module at the expiration of a set time window ifthe Wi-Fi signal monitor detects a second Wi-Fi signal level equal to orgreater than V_(th1) during the set time window.
 21. A mobilecommunication device comprising: a cellular communication module adaptedto communicate with a wireless cellular network; a Wi-Fi communicationmodule adapted to communicate with an access point associated with aWi-Fi network using a VoIP protocol; and a switching circuit adapted toautomatically switch operation between the cellular communication moduleand the Wi-Fi communication module, wherein said tear-down signal causesthe cellular communication module to go into a sleep mode.
 22. A mobilecommunication device comprising: a cellular communication module adaptedto communicate with a wireless cellular network; a Wi-Fi communicationmodule adapted to communicate with an access point associated with aWi-Fi network using a VoIP protocol; and a switching circuit adapted toautomatically switch operation between the cellular communication moduleand the Wi-Fi communication module, wherein upon activation of thetimer, the network switching circuit causes the Wi-Fi communicationmodule to change state from a sleep mode to an active mode.
 23. Themobile communication device of claim 22 further comprising: a timer; anda Wi-Fi signal monitor; wherein if the Wi-Fi signal monitor detects thata first Wi-Fi signal level is greater than a first predefined thresholdvalue V_(th1), the timer is activated to establish a first time windowof a first predefined size T₁, wherein if during T₁ the Wi-Fi signalmonitor detects that a second Wi-Fi signal level is equal to or greaterthan V_(th1), at the expiration of the first time window, the switchingcircuit sends a tear-down signal to the cellular communication moduleand a link-up signal to the Wi-Fi communication module, wherein saidtear-down signal causes the cellular communication module to discontinuehandling a previously established cellular communication and whereinsaid link-up signal causes the Wi-Fi communication module to handle thepreviously established cellular communication via a VoIP network.
 24. Amobile communication device comprising: a cellular communication moduleadapted to communicate with a wireless cellular network; a Wi-Ficommunication module adapted to communicate with an access pointassociated with a Wi-Fi network using a VoIP protocol; and a switchingcircuit adapted to automatically switch operation between the cellularcommunication module and the Wi-Fi communication module, wherein if theWi-Fi signal monitor detects that the Wi-Fi signal level is below asecond predefined threshold value V_(th2), the timer is activated toestablish a second time window of a second predefined size T₂, whereinif during time window T₂, the Wi-Fi signal monitor detects that theWi-Fi signal level is equal to or greater than a third predefinedthreshold value V_(th3), a previously established VoIP communicationcontinues without switching, wherein V_(th3) is smaller than V_(th2).25. The mobile communication device of claim 24 wherein if during timewindow T₂, the Wi-Fi signal monitor detects that the Wi-Fi signal levelis smaller than V_(th3), upon expiration of time window T₂, the timerunit is activated to establish a third time window of a third predefinedsize T₃, where T₃ is smaller than T₂, wherein if during time window T₃the Wi-Fi signal monitor detects that the Wi-Fi signal level is smallerthan V_(th3), the switching circuit sends a tear-down signal to theWi-Fi communication module and a link-up signal to the cellularcommunication module, wherein said tear-down signal causes the Wi-Ficommunication module to discontinue handling a previously establishedVoIP communication and wherein said link-up signal causes the cellularcommunication module to handle the previously established VoIPcommunication via a cellular network.
 26. The mobile communicationdevice of claim 25 wherein at the expiration of the predefine timeperiod T₂, the network switching circuit causes the cellularcommunication module to change state from a sleep mode to an activemode.